This invention relates to methods and apparatus for embedding code words in an audio signal and to methods and apparatus for synchronising a device to the audio signal using the embedded code words. The invention has particular, but not exclusive relevance to communications systems in which a portable user device, such as a computer tablet or a cellular telephone, is provided with data via an acoustic data channel.
The applicant has proposed a number of acoustic data communication systems. Examples of earlier systems can be found in WO2008/145994, WO2009/144470 and WO02/45273, which describe systems in which hidden data can be transmitted to a portable user device (such as a cellular telephone) within the audio of a television or radio programme that may be being delivered over a broadcast channel or from a local storage device (DVD etc). In the present context, the data is hidden in the sense that it is encoded in order to try to hide the data in the audio so that it is not obtrusive to the user. As those skilled in the art will appreciate, the acceptable level of audibility of the data will vary depending on the application and the user involved. Various techniques are described in these earlier patent applications for encoding the data within the audio, including spread spectrum encoding, echo modulation, critical band encoding etc. Any of these data hiding techniques may be used in the present invention.